THE INTERNAL EAR IN VERTEBRATES 587 



differences in the character of the stimulations transmitted to 

 the brain. 



Apart from any question of sound analysis, it is well recog- 

 nised that frogs have a very shrewd power of discrimination, 1 

 as they respond with the greatest alacrity to the croaking of 

 their own species, whilst to other sounds even of the most 

 varied and alarming or seductive description they may remain 

 to all appearances deaf. 



Among reptiles the cochlea makes great strides towards per- 

 fection. In the lowest forms it is scarcely present at all, in 

 crocodiles it is practically the same as in a bird. In all, however, 

 even the lowest, there is one very significant change : the sense 

 organ of the pars basilaris lies on the basilar membrane. This 

 is a difference that marks a distinct step towards the perfection 

 of the cochlea and possibly means the initiation of an entirely 

 new mode of stimulation. In any case, whatever the precise 

 physiological meaning, it is one of the distinctive anatomical 

 characters of the organ of Corti as opposed to an ordinary 

 sense organ of the labyrinth that it should rest actually upon a 

 thin, tense and probably vibratile membrane in the direct path 

 of vibrations passing across the scala media, not upon the sur- 

 rounding thick and stationary wall of the labyrinth. 



In the further evolution of the cochlea, two tendencies may 

 be observed — one leading towards an increase in the length and 

 complexity of the scala media, particularly as concerns the 

 basilar membrane and the sense organ, the other making for 

 greater simplicity 2 of the perilymph spaces. The tendency to 

 elaboration results in an increase in the number and a regular 

 variation in the size of the sensory elements and of the various 

 structures associated with them ; the tendency to simplification 

 of the perilymph spaces ensures that the sense organ is sus- 

 pended in the direct and unimpeded path of movements 

 originating at the fenestra ovalis. 



In reptiles the cochlear canal or pars basilaris lagenae can 

 be found in any condition between that of a lowly amphibian 

 and that of a bird. In different genera of snakes and lizards it 

 and its sense organ show a progressive increase in length cul- 

 minating in the tubular and slightly twisted cochlea of the 



1 Courtis, Am. Nat. 41, 1907, p. 677; Yerkes, Jour, Comp. Neurol. 15, 1905, 



P. 279- 



3 Gray, Proc. Roy. Soc. 80, 1908, p. 507. 



